In the extinction measuring method, a light beam is transmitted along a measurement section which is accessible to ambient air potentially including smoke, and a sensor signal is compared with a value which corresponds to the absence of smoke in the measurement section. As both scattering and absorption of light by smoke particles contribute to light attenuation or extinction, and as light is scattered by bright particles and absorbed by dark particles, the extinction measuring method has relatively uniform sensitivity to different types of smoke particles and is equally suitable for the detection of smouldering fires (bright particles) and open fires (dark particles).
Smoke detectors operating in accordance with the extinction principle are used mainly for monitoring long measurement sections, e.g. in tunnels or warehouses, where they include separate components which are accommodated in separate housings. One housing includes a light source and a light receiver, and the other has a reflector which reflects the beam emitted from the light source back onto the receiver. An electrical signal from the receiver is compared with a predetermined alarm threshold value, e.g. corresponding to 4%/m extinction or 96%/m transmittance of a reference transmission effected at a reference time.
When the extinction measuring method is employed in spot detectors, i.e. smoke detectors accommodated in a single housing, the measurement section is much shorter, and greater sensitivity is required of the transmission measurement. For example, for a 10-cm measurement section, an alarm threshold of 4%/m corresponds to transmission of 99.6% as compared with the reference transmission. If transmission values below the alarm threshold are to be triggered, values of e.g. 99.96% transmission must be detectable, which requires a very high degree of stability of the electronic, optoelectronic and mechanical components of the detector.
To improve detector stability it is known to use a second light receiver for the reference measurement of the light source intensity, whereby light intensity changes can be detected. Also, a second light source can be used so that determination of a measurement value does not depend on the sensitivity of the light receivers. A typical arrangement of this type takes the form of an optical bridge including two light sources and two light receivers, with light from each of the light sources being directed to each of the receivers. Such optical bridges are disclosed in U.S. Pat. No. 4,017,193 and Swiss Patent Document A-643061, for example.
The disclosed optical bridges are based on the assumption that light emitted from the light source is uniformly distributed for passage across the respective air sections to the light receivers, which assumption is valid only in rare, ideal situations. In practice, contamination of the device, temperature fluctuations, and especially changes in the emission characteristics of the light sources will change the distribution of light intensities between the two air sections to an extent which may mimic a change in air transmittance.